8 THE PKOTEIN ELEMENT IN NUTRITION 



Further, it is known that certain substances formed in certain 

 organs are absolutely necessary in the maintenance of health, 

 as, for instance, the substances elaborated by the suprarenals, 

 thyroid, pituitary body, etc. Adrenalin is derived from an 

 aromatic precursor, and Hopkins believes that the suprarenal 

 requires a constant supply of some one of the aromatic groups of 

 the protein molecule to serve as an indispensable basis for its 

 elaboration. If such a constant supply is not available, as, for 

 instance, in starvation, a precursor would have to be provided by 

 the breaking down of some of the tissues of the body. 



In connection with the necessity of the body being provided 

 with the particular combinations of units suited to its needs, it 

 is important to bear in mind that the proteins contained in food 

 whether animal or vegetable are different in composition from 

 those found in the tissues of the body. The products obtained 

 from various proteins on hydrolysis show very marked differences 

 in the proportions of the various amino-acids. These differences 

 are well brought out in the table opposite. 



It is evident, therefore, that in protein metabolism the body 

 selects the particular combinations it requires, and builds them 

 up into its own peculiar types of nitrogenous matter, and that 

 the type of protein supplied in the food is of little importance, so 

 long as it is capable of furnishing a sufficiency of the particular 

 types of amino-acid needful in the synthesis of tissue proteins. 

 The feeding experiments with gelatin, already referred to, is a 

 good instance of the importance of the presence of special com- 

 binations of molecules in the food. The role of protein in the 

 diet cannot be undertaken by gelatin because, although gelatin 

 can be broken down into different amino-acids, such as arginine, 

 lysine, histidine, etc., it is unable to provide tyrosine, trypto- 

 phane, or cystine, all of which are essential in the synthesis of 

 tissue proteins. The addition of these bodies to a diet of gelatin 

 enhances its value enormously, so that it may even take the place 

 of true protein. Further proof of the power the tissues possess 

 of forming their own particular types of protein, whatever the 

 diet may be, is forthcoming in the experiment of Abderhalden, 

 who bled a horse until a large quantity of the serum-protein was 

 lost. The animal was then fed on gliadine, which contains four 

 times as much glutamic acid as horse's serum-protein ; in spite 

 of the absorption of glkdine the serum-protein of the horse's blood 

 remained the same. The same holds good for the vegetable 



